1. Field
The present invention relates to wireless telecommunications. More particularly, the present invention relates to a novel method for facilitating handoffs between digital base stations with different spectral capabilities.
2. Background
In a code division multiple access (CDMA) spread spectrum communication system, a common frequency band is used for communication with all base stations within that system. An example of such a system is described in the TIA/EIA Interim Standard IS-95-A entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, incorporated herein by reference. The generation and receipt of CDMA signals is disclosed in U.S. Pat. No. 4,901,307 entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS USING SATELLITE OR TERRESTRIAL REPEATERS” and in U.S. Pat. No. 5,103,459 entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM” both of which are assigned to the assignee of the present invention and incorporated herein by reference.
Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high rate pseudonoise (PN) code. A PN code is used to modulate signals transmitted from the base stations and the remote stations. Signals from different base stations can be separately received at the receiving station by discrimination of the unique time offset that is introduced in the PN codes assigned to each base station. The high rate PN modulation also allows the receiving station to receive a signal from a single transmission station where the signal has traveled over distinct propagation paths. Demodulation of multiple signals is disclosed in U.S. Pat. No. 5,490,165 entitled “DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS” and in U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, both of which are assigned to the assignee of the present invention and incorporated herein by reference.
U.S. Pat. No. 5,101,501 entitled “METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,” and U.S. Pat. No. 5,267,261 entitled “MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM,” both of which are assigned to the assignee of the present invention and are incorporated herein by reference, disclose a method and system for simultaneous communication between a remote station and more than one base station, known as soft handoff. Further information concerning handoff is disclosed in U.S. Pat. No. 5,101,501, entitled “METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM”, U.S. Pat. No. 5,640,414, entitled “MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM”, and U.S. Pat. No. 5,625,876 entitled “METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION,” each of which is assigned to the assignee of the present invention and incorporated in its entirety herein by this reference. The subject matter of U.S. Pat. No. 5,625,876 concerns what is known in the art as “softer handoff.” For the purposes of this document, the term “soft handoff” is intended to include both “soft handoff” and “softer handoff.”
If a remote station travels outside the boundary of the system with which it is currently communicating, it is desirable to maintain the communication link by transferring the call to a neighboring system, if one exists. The neighboring system may use any wireless technology, examples of which are CDMA, NAMPS, AMPS, TDMA or FDMA. If the neighboring system uses CDMA on the same frequency band as the current system, an inter-system soft handoff can be performed. In situations where inter-system soft handoff is not available, the communication link is transferred through a hard handoff where the current connection is broken before a new one is made. Examples of hard handoffs are those from a CDMA system to a system employing an alternate technology or a call transferred between two CDMA systems which use different frequency bands (inter-frequency hard handoff).
Inter-frequency hard handoffs can also occur within a CDMA system. For example, a region of high demand such as a downtown area may require a greater number of frequencies to service demand than the suburban region surrounding it. It may not be cost effective to deploy all available frequencies throughout the system. A call originating on a frequency deployed only in the high congestion area must be handed off as the user travels to a less congested area. Another example is that of a microwave or other service operating on a frequency within the system's boundaries. As users travel into an area suffering from interference from the other service, their call may need to be handed off to a different frequency.
Handoffs can be initiated using a variety of techniques. Handoff techniques, including those using signal quality measurements to initiate handoff, are found in U.S. Pat. No. 5,697,055 entitled “METHOD AND APPARATUS FOR HANDOFF BETWEEN DIFFERENT CELLULAR COMMUNICATIONS SYSTEMS”, issued Dec. 9, 1997, assigned to the assignee of the present invention and incorporated herein by reference. Further disclosure on handoffs, including measurement of round-trip signal delay to initiate handoff, is disclosed in U.S. Pat. No. 5,848,063, entitled “METHOD AND APPARATUS FOR HARD HANDOFF IN A CDMA SYSTEM”, issued Dec. 8, 1998, assigned to the assignee of the present invention and incorporated herein by reference. Handoffs from CDMA systems to alternate technology systems are disclosed in U.S. Pat. No. 5,594,718 ('306 application) entitled “METHOD AND APPARATUS FOR MOBILE UNIT ASSISTED CDMA TO ALTERNATIVE SYSTEM HARD HANDOFF”, issued Jan. 14, 1997, assigned to the assignee of the present invention and incorporated herein by reference. In the '306 application, pilot beacons are placed at the boundaries of the system. When a remote station reports these pilots to the base station, the base station knows that the remote station is approaching the boundary.
When a system has determined that a call should be transferred to another system via hard handoff, a message is sent to the remote station directing it to do so along with parameters that enable the remote station to connect with the destination system. The system has only estimates of the remote station's actual location and environment, so the parameters sent to the remote station are not guaranteed to be accurate. For example, with beacon aided handoff, the measurement of the pilot beacon's signal strength can be a valid criterion for triggering the handoff. However, the appropriate cell or cells in the destination system which are to be assigned to the remote station (known as the Active Set) are not necessarily known.
The cdma2000 cellular telephone standard proposal issued by the Telecommunications Industry Association (TIA), entitled “cdma2000 Series TIA/EIA/IS-2000,” published in August of 1999, hereinafter referred to as cdma2000, uses advanced signal processing techniques to provide efficient and high quality phone service, and is incorporated herein by reference. For example, a cdma2000 compliant cellular telephone system utilizes decoding, error detection, forward error correction (FEC), interleaving, and spread spectrum modulation in order to make more efficient use of the available radio frequency (RF) bandwidth, and to provide robust connections. In general, the benefits provided by cdma2000 include longer talk time and fewer dropped calls when compared to other types of cellular telephone systems.
In the world of cellular telecommunications, those skilled in the art often use the terms 1G, 2G, and 3G. The terms refer to the generation of the cellular technology used. 1G refers to the first generation, 2G to the second generation, and 3G to the third generation. 1G is used to refer to the analog phone system, known as an AMPS (Advanced Mobile Phone Service) phone systems.
2G is commonly used to refer to the digital cellular systems that are prevalent throughout the world, and include cdmaOne, Global System for Mobile communications (GSM), and Time Division Multiple Access (TDMA). cdmaOne, based on a Code Division Multiple Access (CDMA) technology, refers to a digital cellular system that adheres to the family of IS-95 standards. 2G systems can support a greater number of users in a dense area than can 1G systems.
3G is commonly used to refer to the digital cellular systems currently being developed. 3G systems include cdma2000 and Wideband-CDMA (W-CDMA). 3G systems promise higher peak data transfer rates than do their 2G counterparts. Additionally, many 3G systems can support a greater number of users than can 2G systems.
Spreading Rate 3 versions of cdma2000, hereinafter referred to as 3X, use a frequency band of 3.75 megahertz (MHz), comprised of three 1.25 MHz chunks, while the Spreading Rate 1 versions of cmda2000, hereinafter referred to as 1X, uses a frequency band having a width of 1.25 MHz. Wherein 1X is a single carrier protocol, 3X is a multi-carrier protocol. As would be known to one skilled in the art, a single carrier protocol transmits data in a single frequency band, while a multi-carrier protocol, transmits data in multiple [single carrier] frequency bands. For instance, whereas 1X transmits data in a single 1.25 MHz frequency bands, 3X transmits data in three 1.25 MHz frequency bands. The modulation techniques performed by multi-carrier systems, hereinafter referred to as a multi-carrier modulation techniques, differ from those performed by single carrier systems, hereinafter referred to as single carrier modulation techniques. Although the examples of 1X and 3X are used as single carrier and multi-carrier protocols respectively, the invention is not limited to 1X and 3X protocols, and applies equally well to any system that is comprised of both single carrier and multi-carrier base stations, such as may be the case in future versions of W-CDMA that are attempting to support higher data transmission rates.
Any given cdma2000 system need not support 1X (version 1X of cdma2000) exclusively or 3X (version 3X of cdma2000) exclusively. A version of cdma2000 that uses a multi-carrier 3.75 MHz spectrum on the forward link, the wireless link that delivers data from the base station to the remote station, but uses a single carrier 1.25 MHz spectrum on the reverse link, the wireless link that delivers data from the remote station to the base station is described in U.S. patent application Ser. No. 09/382,438 entitled “Method and Apparatus Using a Multi-Carrier Forward Link in a Wireless Communication System” assigned to the assignee of the present invention and incorporated herein by reference. A system such as this, that uses multi-carrier capabilities on the forward link, but uses single carrier capabilities on the reverse link, is hereinafter referred to as a hybrid system. A 3X/1X system is merely used as an example, and a hybrid system is not limited to such an embodiment. A block diagram of an exemplary hybrid system is shown in FIG. 1.
FIG. 1 is a block diagram of an exemplary simplified cellular telephone system that uses multi-carrier transmission on the forward link and single carrier transmissions on the reverse link, wherein a 3X protocol is used on the forward link and a 1X protocol is used on the reverse link. Remote stations such as remote stations 110 (typically cellular telephones, personal digital assistants (PDAs) with wireless capabilities, or laptop computers with wireless capabilities) are located among base stations 120. The remote stations 110a and 110b are in an active mode and are therefore interfacing with at least one base station 120 using radio frequency (RF) signals modulated in accordance with the CDMA signal processing techniques. A system and method for modulating RF signals in accordance with CDMA modulation is described in U.S. Pat. No. 5,103,459 entitled “System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System” assigned to the assignee of the present invention and previously incorporated herein by reference. The other remote stations 110 are in standby mode and are therefore monitoring either a full paging channel for page messages indicating a request to communicate, or they are monitoring a quick paging channel for indicator bits indicating whether a message is expected on a full paging channel.
Each given base station 120 that is in active mode with at least one remote station 110 transmits data to remote stations 110 across three frequency bands f1, f2, f3, and receives data from remote stations 110 in a single frequency band f4. Each frequency band f1, f2, f3, and f4 has the same bandwidth. Frequency bands f1, f2, and f3 are adjacent frequency bands. For instance, if in the example of the hybrid system each band f1 was the frequency band 1900 MHz–1901.25 MHz, then f2 would be 1901.25 MHz–1902.5 MHz, and f3 would be 1902.5 MHz–1903.75 MHz. Thus, the adjacent frequency bands take up the spectrum 1900 MHz–1903.75 MHz in this example. In such a case, f4 would be a 1.25 MHz band located outside of that frequency range. For example, f4 could be located at 1820 MHz–1821.25 MHz.
Base stations 120 connect to a Base Station Controller (BSC) 114. Base Station Controller 114 controls base stations 120, and exchanges information packets between Mobile Switching Center (MSC) 116 and base stations 120. Mobile Switching Center 116 exchanges information packets between Public Switched Telephone Network 118. In other embodiments, different switches, such as a packet data serving node (PDSN), can be connected to the system. A cellular telephone system can contain more than one Base Station Controller 114 and more than one Mobile Switching Center 116, or conversely, in decentralized systems, such as those disclosed in U.S. Pat. No. 6,215,779, entitled “DISTRIBUTED INFRASTRUCTURE FOR WIRELESS DATA COMMUNICATIONS”, issued Apr. 10, 2001, assigned to the assignee of the present invention, and incorporated by reference herein, Base Station Controller 114 or Mobile Switching Center 116 may be absent from the cellular telephone system.
FIG. 2 is a block diagram of an exemplary simplified cellular telephone system that uses single carrier transmissions on the forward link and single carrier transmissions on the reverse link. Remote stations such as remote stations 210 (typically cellular telephones) are located among base stations 220. The remote stations 210a and 210b are in an active mode and are therefore interfacing with at least one base station 220 using radio frequency (RF) signals modulated in accordance with the CDMA signal processing techniques. The other remote stations 210 are in standby mode and are therefore monitoring either a full paging channel for page messages indicating a request to communicate, or they are monitoring a quick paging channel for indicator bits indicating whether a message is expected on a full paging channel.
Each given base station 220 that is in active mode with at least one remote station 210, transmits data to remote stations 210 across a single frequency band f1, and receives data from remote stations 210 in a single frequency band f2. Each frequency band f1 and f2 has the same bandwidth. Frequency bands f1 and f2 can be offset by a predefined amount. If an amount of 80 MHz is used, and f1 is at the frequency band 1900 MHz–1901.25 MHz, then f2 could be located at 1820.00 MHz–1821.80 25 MHz.
Base stations 220 connect to a Base Station Controller 114. Base Station Controller 114 controls base stations 220, and exchanges information packets between Mobile Switching Center 116 and base stations 220. Mobile Switching Center 116 exchanges information packets between Public Switched Telephone Network 118. In other embodiments, different switches, such as a packet data serving node (PDSN), can be connected to the system. A cellular telephone system can contain more than one Base Station Controller 114 and more than one Mobile Switching Center 116, or conversely, in decentralized systems, such as those disclosed in U.S. Pat. No. 6,215,779 entitled “DISTRIBUTED INFRASTRUCTURE FOR WIRELESS DATA COMMUNICATIONS”, issued Apr. 10, 2001, applied for by the applicant of the present invention, and incorporated by reference herein, Base Station Controller 114 or Mobile Switching Center 116 may be absent from the cellular telephone system as separate entities, but rather may be integrated into the base stations themselves.
FIG. 3 is a block diagram of an exemplary simplified cellular telephone system that uses multi-carrier transmissions on the forward link and multi-carrier transmissions on the reverse link. Remote stations such as remote stations 310 (typically cellular telephones) are located among base stations 320. The remote stations 310a and 310b are in an active mode and are therefore interfacing with at least one base station 320 using radio frequency (RF) signals modulated in accordance with the CDMA signal processing techniques. The other remote stations 310 are in standby mode and are therefore monitoring either a full paging channel for page messages indicating a request to communicate, or they are monitoring a quick paging channel for indicator bits indicating whether a message is expected on a full paging channel.
Each given base station 320 that is in active mode with at least one remote station 310, transmits data to remote stations 310 across three frequency bands, f1, f2, f3, and receives data from remote stations 310 across three frequency bands f4, f5, f6. Each frequency band f1, f2, f3, f4, f5, f6 has the same bandwidth. Frequency bands f1, f2, and f3 are adjacent frequency bands. For instance, if in the example of the hybrid system each band f1 was the frequency band 1900 MHz–1901.25 MHz, then f2 would be 1901.25 MHz–1902.5 MHz, and f3 would be 1902.5 MHz–1903.75 MHz. Thus the adjacent frequency bands take up the spectrum 1900 MHz–1903.75 MHz in this example. Likewise, frequency bands f4, f5, and f6 are adjacent to one another. Frequency bands f1 and f4 can be offset by a predefined amount. If an amount of 80 MHz is used, and f1 begins at the frequency 1900 MHz, then reverse link bands f4, f5, f6, could take up the 3.75 MHz spectrum located between 1820 MHz–1823.75 MHz.
Although a carrier could upgrade its entire network from a single carrier system to a multi-carrier system all at once, this is often undesirable in light of costs. Economically, it may be more desirable for a carrier to put multi-carrier capabilities in a few areas of their network that will benefit the most from multi-carrier capabilities, and then slowly roll out multi-carrier capabilities into other areas of its network over time. During such a gradual rollout, some of the base stations in the system will be multi-carrier protocol compliant while others will not. Likewise, some carriers may find it desirable to upgrade a portion of its network to support a multi-carrier protocol, without having the intent of ever upgrading the remaining portion of its network to a multi-carrier protocol.
What is needed is a method and apparatus for performing handoff in a wireless telecommunication system that contains digital base stations, some of which comply with a multi-carrier protocol and some of which are not multi-carrier compliant.